Changed code a bit and added new features

Code changes:
  - All audio i/o stuff is now in device.*
  - All core related stuff is now in audio_call.c*
  - Multiple devices per stream and vice versa

Features:
  - VAD (Voice* activity detection); energy based.
  - Muting devices
  - Device switching during the call

Wait for core update first.

* By voice i mean any activity that triggers VAD, usually voice.